I have been running 3X Helltooth WDs and 1x buff WD last 2 days, and at Torment 50 the Rift Guardian goes down under 2 seconds. Seriously.

This is because all the pets are hitting a single target with increased damage... However white packs are going down much slower, since the pets tend to split on all targets (and some are just sitting ducks).

But this is probably not the setup I'll play for level 70+ GRs...